Eddy current thermometer

ABSTRACT

A remote, noncontact temperature determination method and apparatus is provided, which is operable to determine the temperature of a conducting member forming a part of or in operative thermal communication with an object of interest. The method comprises the steps of first inducing a closed vortex eddy current ( 28 ) in a conducting member ( 16, 38, 44 ) by subjecting the member ( 16, 38, 44 ) to a magnetic field, such that the corresponding eddy current magnitude changes exponentially over time. A characteristic time constant of the exponential current magnitude changes is then determined, and this is used to calculate the temperature of the object. The apparatus ( 24 ) includes a field transmitting coil ( 14 ) coupled with a waveform generator ( 12 ) for inducing the eddy current ( 28 ), and a field receiving coil assembly ( 18 ) which detects the corresponding magnetic field induced by the eddy current ( 28 ). Using the invention, temperature determinations can be made which are substantially independent of the relative distance and/or angular orientation between the conducting member ( 16, 38, 44 ) and the field receiving coil assembly ( 18 ).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional application Ser. No.61/279,229, filed Oct. 19, 2009. This prior application is incorporatedby reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is broadly concerned with methods and apparatusfor the determination of temperatures of an object by utilizing magneticfield-induced eddy currents in a conducting member forming a part of orin operative thermal communication with the object. More particularly,the invention is concerned with remote, noncontact temperaturedetermination methods and apparatus wherein characteristic timeconstants of the eddy currents are ascertained and used in calculatingthe temperature of the conducting member.

2. Description of the Prior Art

There is a strong demand in modern industry and other fields for remote,noncontract temperature sensing devices. This demand is not satisfied byknown infrared thermometers, given that these require an unobstructedline of sight for operation. The most advanced RFID circuits withintegrated temperature sensing elements tend to be expensive and requirereliable information transfer conditions, (e.g., RF) which restrictstheir use.

It is well known that alternating magnetic fields produce electromotiveforces that excite eddy currents in electrically conductive objects.These currents are in the form of closed vortices, with the shape andspace distribution of these vortices being defined by the alternatingmagnetic field pattern in space and time, and by the conductivity andmagnetic properties of the conductive objects. Such closed vortices areconsidered as closed contours with current flow characterized by certaininductance and resistance values.

Attempts have been made in the past to utilize the eddy currentphenomenon in order to measure the temperatures of the conductiveobjects. These efforts have not been fully successful, however. U.S.Pat. No. 5,573,613 describes a method an apparatus for sensing atemperature of a metallic bond line (susceptor) in an inductive weldingprocess employing a conductive susceptor at the interface between twoplastic parts. A magnetic work coil generates an alternating magneticfield through the plastic parts and around the susceptor. This in turnheats the susceptor, and the electrical resistance thereof changes as afunction of the thermal coefficient of resistance of the susceptormaterial. Such resistance changes are reflected back as a change in themagnetic coil impedance. An electrical circuit senses the varyingresistances, and such changes are translated into sensed temperatures.The sensed temperatures may then be used to adjust the power to themagnetic work coil, or the speed of travel of the work coil along thebond line. This technique does not require line of sight for operation.However, a significant drawback of this method is the dependence uponwork coil impedance changes, which varies significantly with thedistance between the work coil and the susceptor. Thus, this distancemust be carefully maintained to ensure temperature measurement accuracy.

U.S. Pat. No. 3,936,734 describes remote measurement of conductivitiesand/or temperatures of metal components by means of the eddy currenteffect induced within the metal component by an alternating magneticfield. This magnetic field is produced by an excitation coil driven withalternating current arranged so that its axis perpendicular to thesurface of the metal component. Also, a pair of measuring coils of equalradius are arranged coaxially and symmetrically with respect to theexcitation coil at each end of the latter. The two measuring coils areconnected electrically in series, and the phase angle between thecurrent in the measuring coils and the current in the excitation coil istaken as an indication of the measured variable. In order to reduce theeffect of distance changes, the measuring coils are placed at such adistance from the metal component surface that the phase angle betweenthe excitation coil signal and the measuring signal is maximized. Thismethod is inconvenient in practical use, however, owing to the necessityof mechanically adjusting the distances from the metal component to thesensor coils for each measurement.

See also, JP2000193531A; Ueda et al, Development of Methodology forIn-Service Measurement of Transient Responses of Process Instrument usedin LMFBR, International Corporation and Technology Development Center;Takahira et al., Impedance Variation of a Solenoid Coil Facing a MovingSheet Conductor, Electrical Engineering in Japan, Vol. 103, Issue 3, pp.1-7 (1983); and Keller, A New Technique for Noncontact TemperatureMeasurement of Rotating Rolls, Iron Steel Eng., Vol. 57, No. 5, pp.42-44 (May 1980).

SUMMARY OF THE INVENTION

The present invention overcomes the deficiencies of the prior art eddycurrent temperature measurement techniques, and provides a method andapparatus for remote, noncontact temperature measurement of a conductingmember (e.g., a metal, semiconductor, or liquid electrolyte) forming apart of or in operative thermal communication with an object ofinterest. The methods and apparatus hereof are substantially independentof the distance and/or relative angular orientation between theconducting member and a detecting coil assembly.

Broadly speaking, the invention provides a method and apparatus fordetermining the temperature of an object comprising the steps ofinducing an eddy current in such a conducting member, wherein the eddycurrent is induced by subjecting the conducting member to a magneticfield having a magnitude which varies substantially linearly over time(i.e., no more than about ±30% of true linearity), such that thecorresponding eddy current magnitude changes exponentially over time.Next, a characteristic time constant of the exponential currentmagnitude changes is determined, and the temperature of the object iscalculated using this characteristic time constant.

In preferred forms, a magnetic field transmitting coil driven using atriangular waveform alternating current is employed to induce the eddycurrent in the conducting member, and a receiving coil assembly isprovided to detect the corresponding eddy current-induced magneticfield. The output voltage of the receiving coil assembly is then used todetermine the characteristic time constant. The receiving coil assemblyadvantageously comprises a pair of receiving coils in electrical seriesbut of opposite phases, with the receiving coils located on oppositesides of the field transmitting coil. In order to facilitate thetemperature measurement, the receiving coils are compensated so that, inthe absence of the conducting member, the voltage output from thereceiving coil assembly is zero.

The methods and apparatus of the invention can be used in a variety ofcontexts where remote, noncontact temperature sensing is desired. Forexample, the invention can be employed for determining the temperatureof a food material during heating thereof In such uses, a heating vesselis provided presenting a bottom wall and which is operative to hold foodmaterial. A metallic conducting member, such as an aluminum cone, ispositioned on the bottom wall and has a portion thereof projectingupwardly into the food material. A temperature detecting unit ispositioned in proximity to the heating vessel, and includes a firstassembly operable to induce an eddy current in the conducting member,with the magnitude of the eddy current changing exponentially over time.The unit also includes a second assembly operable to determine acharacteristic time constant of the eddy current magnitude changes, andto calculate the temperature of the conducting member, and hence thefood material, using characteristic time constant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating the remote temperature sensingapparatus and method of the invention;

FIG. 2 is a block diagram of a preferred overall system for remotetemperature measurements in accordance with the invention;

FIG. 3 is a graph illustrating a single period output voltage of areceiving coil assembly in accordance with the invention;

FIG. 4 is a graph illustrating the remote temperature sensing operationof the apparatus of the invention, as compared with simultaneoustemperature sensing using a conventional thermocouple;

FIG. 5 is a schematic view of food material within a conventionalheating pan, and the latter equipped with a conducting member permittingremote, noncontact sensing of the temperature of the food materialduring heating thereof;

FIG. 6 is a perspective view of a conical conducting member useful infood temperature determinations; and

FIG. 7 is a perspective view of a circular, rounded-shoulder conductingmember also useful in food temperature determinations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 2 schematically illustrates anoncontact, remote temperature sensing system 10 in accordance with theinvention. The system 10 includes a wave form generator 12 operablycoupled with a field transmitting coil 14 and preferably designed toprovide a triangular current waveform drive current to the coil 14. Thegenerator 12 and coil 14 serve to create an alternating magnetic fieldwhich penetrates an electrically conductive member 16. The overallsystem 10 further includes a field receiving coil assembly 18 coupledwith a signal processing circuit 20 which may be connected with aninterface 22 for further processing and displaying of the output ofcircuit 20. It will be appreciated that the system components 12, 14,18, 20, and 22 may be designed in a stand alone unit 24 as an integralpart of an instrument or appliance, or as individual components.

The basic operation of system 10 can be understood by a consideration ofFIG. 1. As shown therein, the field transmitting coil 14, driven by thegenerator 12, produces a linearly varying in time magnetic field whichpenetrates the conducting member 16 (in this instance illustrated as aflat plate). Exemplary field lines 26 are shown to illustrate thiseffect. The magnetic field creates a stable electromotive force inducingeddy currents 28 (in the form of closed rings or contours). However,eddy current stabilization does not occur instantaneously, owing to thefinite inductance L and resistance R experienced by the eddy currents28. Rather, the eddy current magnitude I stabilizes according to awell-known exponential law:I(t)=I ₀ exp(−tR/L)  (Equation 1)where t is a time variable, and I₀ is the stable current value.

The eddy currents 28 also generate a corresponding magnetic field whichis sensed by the field receiving coil assembly 18, along with the fieldgenerated by coil 14. In this instance, the assembly 18 includes a pairof receiving coils 30, 32 which are positioned on opposite sides of andcoaxially aligned with the field transmitting coil 14. The coils 30, 32are connected in electrical series, but in opposite phases. Thepositions of the coils 30, 32 relative to the field transmitting coil 14are preferably chosen such that when the conducting member 16 is notpresent, the voltages induced in the coils 30, 32 by the magnetic fieldgenerated by coil 14 are fully compensated, with a resulting outputvoltage signal of zero.

However, when the member 16 is present, the receiving coils 30,32 arerespectively at different distances from the member 16; therefore, thevoltage induced in receiving coil 30 (by a combination of the magneticfields generated by field coil 14 and eddy currents 28) is significantlygreater than that in coil 32. The resulting voltage output signals ofthe coils 30 and 32 are proportional to the rate of change of the eddycurrent-induced magnetic field. This is an exponential function definedby a characteristic time constant τ=L/R from Equation 1.

The eddy current inductance L is defined by the dimensions of thecurrents 28, which in turn are determined by the configuration of themagnetic field created by field transmitting coil 14. The eddy currentresistance R is defined by these same current 28 dimensions and theelectrical conductivity σ of the conducting member 16. Given that thecurrent 28 dimensions are stable, the time constant τ=L/R is thusproportional to the conductivity σ of the material making up member 16.The conductivity σ is a well-defined, monotonous function of thetemperature T of the member 16:τ=Aσ(T)=F(T)  (Equation 2)wherein A is a constant, and the σ(T) conductivity dependence ontemperature is well-known for essentially all existing metals andalloys. The particular value of A for a given member 16 may readily bedetermined, for example by measuring the time constant τ at one knowntemperature, and using the corresponding known a value for thattemperature.

It is thus possible to determine the temperature T of the member 16using the measured eddy current time constant τ:T=F ⁻¹(τ)  (Equation 3)where F⁻¹ is the inverse function of F.

In the event that the relationship between the conductivity a totemperature T is unknown for a given conducting member 16, the F⁻¹ (τ)function may be determined empirically by measuring the time constant τat multiple temperatures, and then curve-fitting the obtained timeconstant data with a polynomial or other appropriate mathematicalfunction.

It is to be understood that the member 16 may be a zone or area of anobject subjected to temperature measurement, or may be a separate memberor body in operative thermal communication with the object. In eithercase, the temperature of the object can be accurately measured.

In principle, a single τmeasurement in which the magnetic fieldmagnitude of the field transmitting coil 14 rises linearly from zero toa certain maximum is sufficient to determine a characteristic value ofthe time constant. However, it may be advantageous to use an alternatingmagnetic field of triangular waveform from the field transmitting coil14 so that a plurality of τmeasurements can be averaged in order toimprove the accuracy of τ. In such a case, it is desirable that ahalf-period of the alternating magnetic field is substantially longerthan the eddy current time constant τ.

Referring now to FIG. 3, a single period example of the voltage outputfrom the field receiving coil assembly 18 is illustrated. The overallsystem 10 was operated at a frequency of 500 Hz, with receiving coil 30placed at a distance of 25 mm from the member 16 (formed of 2012aluminum alloy and having a thickness of 4 mm). The time constant τ wasmeasured to be 134 microseconds.

In another test, a conventional sauce pan was heated and the pantemperature was determined both by the system 10 and a conventionalthermocouple. The sauce pan was formed of stainless steel and had acapacity of 1.5 liter. It was also equipped with an aluminum bottom heatspread disk sandwiched between stainless steel layers. The receivingcoil 30 was placed at a 25 mm distance from the sauce pan bottom. AK-type thermocouple was also secured to the sauce pan bottom by adhesivetape and further pressed by a piece of thick cardboard. The sauce panwas heated from its inside by hot air blower for approximately 40seconds. FIG. 4 depicts temperature vs. time graphs for both the eddycurrent (EC) system 10 of the invention, and the thermocouple (TC)temperatures. It can be seen that while the EC measurements consistentlygive exact, instantaneous temperature values, the TC exhibited a notabledelay in the fast heat up interval. However, in the intervals of slowtemperature changes, both methods exhibited similar accuracies. Itshould also be noted that, in spite of the aluminum heat spread diskbeing cladded between opposing layers of stainless steel, the eddycurrent time constant T was in fact defined by the conductivity σ of thealuminum. This is due to the much lower conductivity of stainless steel(approximately 20 times lower than aluminum) and the small thickness ofthe stainless steel cladding. In essence, the contribution to thereceived signals from the stainless steel was negligible.

The temperature determination methods and apparatus of the invention donot rely upon a line-of-sight orientation between the system 10 and theconducting member 16.

As such, the invention is very useful in numerous fields of application,e.g., repairs in the aerospace industry, control of plastic weldingprocesses, smart cookware, and any other application where remote,noncontact, non line-of-sight temperature determination is advantageous.As noted, the object of interest to be temperature measured need notitself be conductive, but instead a separate conducting member may beused as a remote sensor when placed in thermal communication with thenon-conducting object. Such separate conducting members may be shaped aspieces of conducting foil, small conducting disks, or otherwise.

Another important feature of the present invention resides in the factthat the value of the time constant τ (and thus the corresponding objecttemperature T) is essentially independent of the distance and/or angularorientation between the object 16 and the field receiving coil assembly18.

For instance, a 3 mm thick aluminum disk having a diameter of 25 mm hasbeen tested and found to show good temperature accuracy. The fieldtransmitting coil 14 in this instance was made up of a cylindrical coilhaving a 50 mm outer diameter, a 50 mm height, a resistance of 21 Ohms,and an inductance of 40 mH. The field receiving coil assembly comprisedseparate receiving coils 30, 32 each having a diameter of 72 mm, aheight of 8 mm, and with 250 turns of 0.2 mm copper wire in each coil.The coils were assembled as depicted in FIG. 1. The field transmittingcoil was driven by a triangular current waveform of 500 Hz frequency andan amplitude of 0.5 A. Prior to the test, the two field receiving coilswere mechanically adjusted so that the output signal therefrom in theabsence of a conducting object was equal to zero.

The aluminum disk was then placed at different distances from the fieldreceiving coil 30. The measured time constant value at 24 mm distancewas 150.0±0.1 microseconds. Thereupon, the distance was decreased to 20mm and the measured value was again 150.0±0.1 microseconds. Then thedistance was increased to 28 mm, and the exact same time constant valuewas again determined. The conductivity of aluminum metal making up thedisk varies at 0.4% per ° C. at room temperature. Hence, the temperatureaccuracy of this test was estimated to be ±0.17° C. Subsequently, theangle between the plane of the aluminum disk and the receiving coil 30was altered from zero to ±15°. No changes in the measured time constantvalue were observed. It was thus concluded that the temperaturemeasurement system exhibited virtually no dependence upon the distanceor angular orientation of the disk, and hence the measured disktemperatures are likewise independent of these factors.

The invention is particularly useful in the context of remote,noncontact temperature measurement of foods during heating thereof Forexample, as illustrated in FIG. 5, a conventional stainless steel pan 34has a volume of food material 36 therein. A conical aluminum member 38is also within the pan 34, resting upon the bottom wall 40 thereof Withsuch an assembly, the temperature of the food material 36 may be readilymonitored through use of a system 10 in accordance with the invention.Although a conical member 38 is preferred owing to the fact that it hasa sidewall surface 42 projecting well into the volume of food material36, other shapes may be employed. For example, FIG. 7 illustrates amember 44 which is substantially circular and has a continuous roundedshoulder 46.

Alternately, the pan 34 may be equipped with a conducting member on theouter surface of the bottom wall 40. In this embodiment, the foodtemperature would not be measured directly, but would still provideuseful information about the food temperature. The conducting member maybe in the form of a small disk embedded in or attached to the bottomwall 40.

The invention claimed is:
 1. A method of determining the temperature ofan object, comprising the steps of: inducing an eddy current in aconducting member forming a part of or in operative thermalcommunication with said object, said eddy current induced by subjectingsaid conducting member to a magnetic field generated by a fieldgenerating coil and having a magnitude which varies substantiallylinearly over time, such that the corresponding eddy current magnitudechanges exponentially over time and thereby generates a correspondingeddy current-induced magnetic field; determining a characteristic timeconstant of said eddy current magnitude changes, said timeconstant-determining step comprising the steps of using a receiving coilassembly to detect said corresponding eddy current-induced magneticfield, said receiving coil assembly operable to generate an outputvoltage in response to said eddy current-induced magnetic field, andusing said output voltage to determine said characteristic timeconstant; and calculating the temperature of said object using saidcharacteristic time constant.
 2. The method of claim 1, including thestep of driving said transmitting coil with a triangular waveformalternating current.
 3. The method of claim 1, said receiving coilassembly comprising a pair of receiving coils in electrical series butof opposite phases.
 4. The method of claim 2, including the step ofdriving said transmitting coil using an alternating magnetic fieldhaving a half-period, the half-period being substantially longer thansaid time constant.
 5. Apparatus for determining the temperature of anobject having a conducting member forming a part of or in operativethermal communication with the object, said apparatus comprising: afirst assembly comprising a magnetic field generating coil and a currentgenerator operably coupled with the field generating coil, said fieldgenerating coil operable to induce an eddy current in said member, themagnitude of said eddy current changing exponentially over time, andthereby generating a corresponding eddy current-induced magnetic field;and a second assembly comprising a receiving coil assembly operable todetect said eddy current-induced magnetic field and to generate anoutput voltage in response to the eddy current-induced magnetic field,said second assembly further including a signal processor operable todetermine a characteristic time constant of said eddy current magnitudechanges, and to calculate the temperature of said object using saidcharacteristic time constant.
 6. The apparatus of 5, said currentgenerator operatively coupled with said magnetic field transmitting coilwhich drives the transmitting coil using a triangular waveformalternating current.
 7. The apparatus of claim 6, said triangular waveform having a half-period substantially longer than said time constant.8. The apparatus of claim 5, said receiving coil assembly comprising apair of receiving coils in electrical series but of opposite phases. 9.The apparatus of claim 5, said magnetic field transmitting coil operableto generate a magnetic field whose magnitude changes at a substantiallylinear rate, said receiving coil assembly comprising a pair of receivingcoils in electrical series but of opposite phases, said pair ofreceiving coils respectively located on opposite sides of saidtransmitting coil.
 10. The apparatus of claim 5, said member beingseparable from said object.